1. Field of the Invention
The invention generally relates to cartridges for magnetic disks on which information may be recorded in substantially circular tracks. More particularly, the invention relates to a track number indicator in a disk cartridge for visibly indicating the number of tracks recorded on the magnetic disk.
2. Description of the Prior Art
Magnetic disks have been used for a number of years to record information in various types of devices, such as word processors and computer systems. The most common form of such disks, sometimes referred to as "floppy disks" because of their lack of stiffness, are housed in a cartridge to prevent the flexible disk from being folded or otherwise damaged and to protect the sensitive magnetic surface(s) of the disk from dust and other contaminants. The cartridge includes an exterior housing having at least one access opening through which one or more magnetic heads may be received to gain access to a magnetic surface of the disk. While the disk is rotated within the housing, the head is radially advanced over a magnetic surface of the disk to record or read information in a plurality of concentric tracks on the magnetic surface.
Generally, the magnetic disk cartridges of which I am aware do not include any means for indicating the number of tracks recorded on the magnetic disk. Recently, however, a magnetic disk cartridge having a track number indicator was disclosed in United Kingdom Patent Application GB 2,103,188A, published Jan. 26, 1983. In that cartridge, there is provided a rotatable dial member having a keyhole-shaped opening for receiving a drive pin in a disk recording and/or playback apparatus. The drive pin is incrementally rotated each time a magnetic head in the apparatus is radially advanced over the magnetic surface of the disk by a distance equal to the track pitch. The incrementally rotated pin similarly rotates the dial member relative to a housing of the cartridge to position a key-notch extension of the keyhole-shaped opening, which serves as a pointer, opposite one of a plurality of track numbers imprinted on the outside of the housing. The dial member has circumferential teeth that respectively engage a resilient detent pawl, fixed to the inside of the housing, to releasably retain the dial member in each track number position to which it is rotated. This enables the cartridge to be removed from the recording and/or playback apparatus, and set aside, without losing the correct track number indication. If the cartridge is returned to the apparatus, with the key-notch pointing to a track number other than "0", the drive pin will not be aligned with the notch because the pin is biased to an initial angular position corresponding to the "0" position of the notch. Consequently, the drive pin cannot enter the keyhole-shaped opening and will be depressed by the dial member, thereby energizing a motor in the apparatus to rotate the drive pin and radially advance the magnetic head in synchronization. When the drive pin is rotated into alignment with the key-notch, a spring will urge the pin into the keyhole-shaped opening, de-energizing the motor to leave the head positioned adjacent the same track number on the magnetic disk as that indicated by the notch.
While the disk cartridge disclosed in the published patent application includes a means for indicating the number of tracks recorded on the magnetic disk, such means imposes a significant load on the drive motor in the disk recording and/or playback apparatus because of the continuous engagement between the resilient pawl and the respective teeth on the dial member. The pawl-tooth engagement must be relatively strong to prevent the dial member from being rotated by the drive pin whenever the drive pin is depressed by the dial member and rotated to seek alignment with the key-notch. Conversely, the drive motor must provide sufficient torque to overcome such pawl-tooth engagement in order to rotate the dial member stepwise to its various track number positions.
As the dial member in the disk cartridge is rotated to its respective track number positions, the movement of its teeth flexes the resilient pawl to momentarily remove the pawl from between adjacent pairs of the teeth. Consequently, not only is a significant load imposed on the drive motor, but such load varies in strength, which can present other problems. Moreover, whenever the cartridge is removed from the recording and/or playback apparatus, the pawl-tooth engagement does not positively prevent the dial member from being rotated, such as by inadvertently manipulating it; in which instance, the correct track number indiction could be lost.